1. Field of the Invention
The invention relates to a press and, in particular, a press incorporating separate, defined, selectable pressure stages.
2. Description of the Related Art
Known presses for forming articles from material generally comprise two opposed working surfaces, one of which reciprocally moves between positions spaced from and in pressing contact with the other surface. Pressure is generated between the working surface by a pressing force acting on the one surface being opposed by either a relatively rigid support for the other surface or a counter force acting on the other surface.
Since presses in which the other surface is rigidly supported are relatively inefficient due to the work loss and require substantial strength to support the pressure involved without substantial press deflection, many modern presses incorporate a means for imposing a counter force on the other surface. This is particularly the case where the material being pressed is relatively soft or easy to shape. Presses for such material, such as paper, require a counter force which cooperates with the pressing force to gradually reach the desired pressure acting on the material.
In many such presses the counter force is provided by compression springs, called bolster springs, which support the other surface. The bolster springs resist the pressing force and determine the pressure imposed between the working surfaces. Such presses are shown in U.S. Pat. Nos. 2,377,599 and 3,054,144. A press of this type commonly used for producing paperboard products is the Peerless Forming Machine sold by the Peerless Machine & Tool Corporation.
The known presses using bolster springs have certain disadvantages, particularly where they are used to form paperboard products. One disadvantage is the inability to control when maximum pressure is applied by the bolster springs. In most known presses for paperboard where the working surfaces are complementary dies, as the dies approach complete mating, the spring-generated pressure rises rapidly, often imposing excessive forming pressure before the die mating action has had an opportunity to shape the article. If pressure is fully applied before the paperboard has achieved the required shape, subsequent shaping by the dies will tear the paperboard.
Other disadvantages of spring operated presses include the load imposed by the springs on the press motor necessitating use of higher capacity motors. Moreover, where bolster springs are used, it is difficult and time-consuming to change the forming pressure as product or process changes occur causing significant press down time.
Most known presses provide several pairs of cooperating dies for production of several formed articles for each press cycle. In such presses, each die pair has its own set of bolster springs providing forming pressure. In these multiple across configurations, forming pressure may vary between die pairs anywhere from 30 to 85 percent. Such variation produces articles of varying rigidity and quality. A variety of factors contribute to the undesirable product variation including different spring rates, spring lengths, die setups, and press deflections.
The invention overcomes these and other disadvantages of the prior presses. The press of the invention incorporates a forming system having means for controlling and changing the pressure between the working surfaces during a press cycle. In particular, the invention includes a system permitting selection of two or more counter forces which are applied to the other surface at predetermined times during a press cycle. For example, in a press for forming paperboard articles, the invention provides a dual pressure counter force, a first pressure for shaping the article and a second greater pressure for forming the article.
The dual pressure aspect of the invention provides two advantages. First, since only lower pressure is applied until the dies are fully mated and, therefore, the article has been fully shaped, the tearing of the article which occurs in prior presses is eliminated. Second, the dual pressure eliminates motor overload because high pressure is applied only after the dies are fully mated. When the motor drives a cam, high pressure is applied only during cam dwell and torque transmitted back to the motor is zero.
As each die pair is biased together by a hydraulic cylinder receiving pressurized fluid from a common source, all die pairs in a multiple across die configuration are subjected to the same pressures resulting in consistently uniform product. Moreover, required changes in shaping or forming pressure may be effected with each by merely adjusting the common hydraulic source pressure.
Additional advantages of the invention are set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.